KV metadata in API

The current KV api is nice and simple for getting values:

```python
>>> etcd.get("/key")
b"some-value"
```
It doesn't allow for retrieving the metadata about keys, however.

We need to figure out a nice api for showing all the data: key, create_revision, mod_revision, version, value, lease.

How other projects do it:

python-etcd
----------------

python-etcd returns an object for all reads containing a `.value` attribute to read the actual value:
```python
client.read('/nodes/n2').value
#recursively read a directory
r = client.read('/nodes', recursive=True, sorted=True)
for child in r.children:
print("%s: %s" % (child.key,child.value))
```

kazoo (zookeeper)
-------------------------
kazoo returns two values, the data and the node information:
```python
data, stat = zk.get("/my/favorite")
print("Version: %s, data: %s" % (stat.version, data.decode("utf-8")))
```

python-consul
-------------------

python-consul is pretty ugly:
```python
index, data = consul.Consul().kv.get('foo', index=index)
print data['Value']
```

data is a dict that looks like:
```python
{
"CreateIndex": 100,
"ModifyIndex": 200,
"LockIndex": 200,
"Key": "foo",
"Flags": 0,
"Value": "bar",
"Session": "adf4238a-882b-9ddc-4a9d-5b6758e4159e"
}
```

No source file or test is named. Start by locating the current Python KV get API and compare its return value with the metadata fields listed in the issue; done means an agreed API exposes key, create_revision, mod_revision, version, value, and lease.

Written by the indexing model from the issue text.